1. Field of the Invention
The present invention relates generally to techniques for canceling interfering signals, and more specifically to a sidelobe canceler using an array of sub-antennas for canceling interference introduced through the sidelobes of the main antenna.
2. Description of the Related Art
A prior art sidelobe canceler for a main antenna has an array of sub-antennas connected to multipliers where their output signals are respectively weighted with coefficients supplied from an Applebaum weight controller which operates according to the Applebaum algorithm as described in "Adaptive Arrays", IEEE Transactions on Antennas and Propagation, Vol. AP-24, No. 5, 1976. The outputs of the multipliers are summed together into a sum signal which is subtracted in a subtractor from the output of the main antenna. The subtractor output is supplied to the Applebaum weight controller where it is used as a reference signal to produce the weight coefficients. The Applebaum algorithm is based on the minimum mean square error (MMSE) algorithm and an additional steering vector which represents an estimated arrival direction of the undesired signal. The components of the steering vector are respectively added to the weight coefficients in the correlation loops, so that the directional pattern of the antenna array is oriented toward the source of undesired signal and the signals detected by the array are summed together and used to cancel the undesired signal contained in the output of the main antenna.
The output of the subtractor is further applied to an adaptive equalizer where multipath fading related intersymbol interference is canceled. If the time difference between multipath signals becomes smaller than a certain value, the fading pattern changes from frequency selective mode to flat fading, i.e., a fade occurs over the full bandwidth of the desired signal, making it impossible to equalize the desired signal. In such a situation, diversity reception technique is used.
In addition, a component of the desired signal is also received by the adaptive antenna array and combined with the main antenna signal. Under certain amplitude-phase conditions, the phases of these signals become opposite to each other, canceling part or whole of the desired signal.
U.S. Pat. No. 5,369,412, issued to I. Tsujimoto, Nov. 29, 1994, discloses a sidelobe canceler including an array of sub-antennas, an Applebaum weight controller for controlling the weight coefficients of a first array of multipliers, and a correlator for controlling the weight coefficients of a second array of multipliers according to the output of an adaptive equalizer. The outputs of the sub-antenna array are weighted by the coefficients of the first array of multipliers, and summed together to produce a canceling signal. The outputs of the sub-antenna array are further weighted by the coefficients of the second array of multipliers, summed together to produce a diversity signal. After being combined with the diversity signal and the canceling signal, the main antenna signal is fed into the adaptive equalizer for canceling intersymbol interference.